Greetings,

I am seeing some unexplained performance drop using the above cpus,
using a fairly up-to-date build ( late 145).
Basically, the system seems to be 98% idle, spending most if its time in this 
stack:

              unix`i86_mwait+0xd
              unix`cpu_idle_mwait+0xf1
              unix`idle+0x114
              unix`thread_start+0x8
           455645

Most cpus seem to be idling most of the time, sitting on the mwait instruction.
No lock contention, not waiting on io, I am finding myself at a loss explaining 
what this system is doing. 
(I am monitoring the system w. lockstat, mpstat, prstat).  Despite the 
predominantly idle system, 
I see some latency reported by prstat microstate accounting on the zfs threads.
 
This is a fairly beefy box, 24G memory,  16 cpus.
Doing a local zfs send | receive, should be getting at least 100MB+, 
and I am only getting  5-10MB.  
I see some Intel errata on the 55xx series xeons, a problem with the 
monitor/mwait instructions, that could conceivably cause missed wake-up or 
mis-reported  mwait status.

Anybody else is seeing this?
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